Environmental protection and the control of solid, liquid and gaseous effluents or emissions are key elements in the design of steam generating systems which utilize the heat generated by the combustion of fossil fuels to generate steam. At present, the most significant of these emissions are sulfur dioxide (SO2), oxides of nitrogen (NOx), and airborne particulate.
NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other species generated during combustion. Once the fuel is chosen, NOx emissions are minimized using low NOx combustion technology and postcombustion techniques. If combustion modifications alone are insufficient, postcombustion techniques such as selective noncatalytic reduction (SNCR) or selective catalytic reduction (SCR) systems may be employed. In SNCR or SCR systems, NOx is reduced to nitrogen (N2) and water (H2O) through a series of reactions with a chemical reagent injected into the flue gas. Ammonia and urea are the most commonly used chemical reagents with SNCR systems, while ammonia is most commonly used for SCR systems.
Fluidized bed combustion has distinct advantages for burning solid fuels and recovering energy to produce steam; indeed, the primary driving force for the development of fluidized bed combustors in the United States is reduced S2 and NOx emissions. Typically, this technology can be used to burn high sulfur coals and achieve low SO2 emission levels without the need for additional back-end sulfur removal equipment. Fluidized bed boilers are designed so that the bed operating temperature is between 1500 and 1600° F., resulting in lower NOx emissions. These lower operating temperatures also permit combustion of lower grade fuels (which generally have high slagging and fouling characteristics) without experiencing many of the operational difficulties which normally occur when such fuels are burned.
In CFB reactors or combustors, reacting and non-reacting solids are entrained within a reactor enclosure by an upward gas flow which carries the solids to an exit at an upper portion of the reactor enclosure. There, the solids are typically collected by a primary particle separator, of impact type or cyclone type and returned to a bottom portion of the reactor enclosure either directly or through one or more conduits. The impact type primary particle separator at the reactor enclosure exit typically collects from 90% to 97% of the circulating solids. If required by the process, an additional solids collector may be installed downstream of the impact type primary particle separator to collect additional solids for eventual return to the reactor enclosure.
CFB reactors or combustors are known (see, for example, U.S. Pat. No. 5,343,830 to Alexander et al.) wherein the two or more rows of impingement members located within the furnace or reactor enclosure are followed by a second array of staggered impingement members which further separate particles from the gas stream, and return them via cavity means and particle return means without external and internal recycle conduits.
Both SCR and SNCR systems have been applied to reduce NOx emissions from pulverized coal fired steam generating systems. SNCR systems have also been applied to fluidized bed steam generators, and it has been proposed to combine a CFB steam generator for petroleum coke firing with an SCR system.